About optical pyrometer
The optical pyrometer is designed to
measure temperatures where the peak radiation emission is in the red
part of the visible spectrum, i.e. where the measured body glows a
certain shade of red according to the temperature. This limits the
instrument to measuring temperatures above 600°C. The instrument
contains a heated tungsten filament within its optical system. The
current in the filament is increased until its color is the same as
the hot body: under these conditions the filament apparently
disappears when viewed against the background of the hot body.
Temperature measurement is therefore obtained in terms of the current
flowing in the filament. As the brightness of different materials at
any particular temperature varies according to the emissivity of the
material, the calibration of the optical pyrometer must be adjusted
according to the emissivity of the target. Manufacturers provide
tables of standard material emissivities to assist with this.
The inherent measurement inaccuracy of an optical pyrometer is ±5°C.
However, in addition to this error, there can be a further
operator-induced error of ±10°C arising from the difficulty in judging
the moment when the filament 'just' disappears. Measurement accuracy
can be improved somewhat by employing an optical filter within the
instrument which passes a narrow band of frequencies of wavelength
around 0.65 µm corresponding to the red part of the visible spectrum.
The instrument cannot be used in automatic temperature control schemes
because the eye of the human operator is an essential part of the
measurement system. The reading is also affected by fumes in the path
of sight. Because of these difficulties and its low accuracy, the
optical pyrometer is being rapidly overtaken by hand-held radiation
pyrometers in popularity, although the instrument is still widely used
in industry for measuring temperatures in furnaces and similar